1. Field of the Invention
The present invention relates to a cathode material for a lithium secondary batteries, and more particularly, to a lithium-cobalt-manganese oxides having a layered structure providing a superior structural safety and a high discharge capacity, and the synthesis of the same.
2. Description of the Related Art
Presently, one of a typical commercialized cathode material for the lithium secondary batteries is the lithium-cobalt oxide (LiCoO2). With its high discharge voltage high discharge capacity of 140˜160 mAh/g, and the characteristics of stable recharge and discharge ability, the lithium-cobalt oxide is used in most commercial secondary lithium batteries. However, since the lithium-cobalt oxide can cause environmental contamination and its high cost, many researches have been endeavored to find a substitute material for this electrode.
Another cathode material for the lithium secondary batteries according to the conventional art includes a lithium-nickel oxide (LiNiO2) and a lithium-manganese oxide (LiMn2O4). The lithium-nickel oxide has the advantage of low raw material cost and a large usable voltage capacity, which is in the range of 160˜180 mAh/g depending on the synthesis method, but it has been known that there is a chemical reaction between the lithium-nickel oxide and the electrolyte occurs when discharge/recharge is repeated consecutively, which can cause a safety problem. The lithium-manganese oxide, however, owing to the low discharge capacity compare to other cathode materials and its lower conductivity, is seldom used for commercial purposes.
The researches regarding the lithium-cobalt-manganese oxides have been performed toward improving the characteristic of recharge/discharge by partial substitution of the cobalt ion in the spinel structured LiMn2O4 (Refer to the Korean Patent No. 2002-0016477). However, the lithium-cobalt-manganese oxides based on LiMn2O4 have the discharge capacity of 120˜130 mAh/g or less, which is almost the same discharge capacity of the LiMn2O4 or even lower.
Recently, there have been studies to develop a cathode material using the solid solution of the LiCoO2 and Mn group oxides (LiMnO2 or Li2MnO3) having a stable layered structure. Especially, K. Numata et al. have synthesized LiCoO2 and Li2MnO3, and reported the characteristic of the oxide solid solution of the same (Refer to the Solid State Ionics 117(1999) 257–263). Here, under the assumption that the oxidation state of Co is 3+, Li is 1+, and Mn is 4+, a compound having following formula can be formed: Li[CoxLi(1/3−x/3)Mn(2/3−2x/3)]O2. However, in this formula Li[CoxLi(1/3−x/3)Mn(2/3−2x/3)]O2, K. Numata et al. have investigated mainly the composition region 0.7<X<1, where Co is filled to most of all the transition metal locations, but did not investigated 0.05<X<0.5 composition region. Also, the oxides were synthesized by a solid phase reaction using the carbonate source at temperature of 900˜1,000° C. However, due to the difficulty of the synthetic reaction, an excessive amount of lithium oxide was added. In this way, the synthetic reaction was possible but this method has difficulty in controlling the particle size and requires a large amount of lithium oxide. Also, the discharge capacity of the manufactured cathode material powder in the region of the 0.7<X<1 can not be achieved beyond the range of 130˜140 mAh/g which is not bigger than that of the LiCoO2.